1. Field of the Invention
The present invention relates to an ink jet printing apparatus and an ink jet printing method capable of forming color images by ejecting different kinds of ink from print heads according to print data transferred from a print data source.
The present invention is applicable to any apparatus that uses print mediums such as paper, cloth, leather, nonwoven cloth, OHP sheets and even metals. Examples of applicable apparatus include office automation equipment, such as printers, copying machines and facsimile machines, and industrial manufacturing equipment.
2. Description of the Related Art
Today, with office automation equipment such as personal computers and word processors in widespread use, a variety of printing apparatus and printing methods have been developed to record information entered from these equipment onto the above-described printing mediums. As an information processing capability of office equipment in particular continues to advance, there is a growing trend toward video information going color. In response to this trend, an increasing number of printing apparatus used to output the processed information also are being provided with a color printing capability.
Printing apparatus capable of producing color images are available in various types in terms of cost and function, from an inexpensive one with relatively simple functions to a multifunctional one that can select a desired printing speed and print quality according to the kind of image to be printed and the purpose of use.
Ink jet printing apparatus have features of, for example, low noise, low running cost, small size and ease with which color printing can be introduced, and thus are in wide use with printers, copying machines and facsimile.
Color ink jet printing apparatus generally perform a color printing by using three color inks, cyan, magenta and yellow, or four color inks including black in addition to the three primary inks.
The printing of color images is normally performed according to print data transferred from a host computer or the like. The transfer of print data, for example, is carried out as follows.
In a serial printing apparatus that performs printing by scanning a print head, image information for each color is transferred for each raster or for each line made up of a plurality of rasters, the rasters extending in a main scan direction (hereinafter also referred to as a raster direction) in which the print head is moved. That is, the Y/M/C/Bk image data for the same raster or the same line are transferred and this is followed by the Y/M/C/Bk image data for the next raster or line being transferred.
The most common print data transfer specification is Centronics which transfers data in parallel. The currently available Centronics specification is for one-way data transfer from the host computer to the printing apparatus. A two-way Centronics specification for bidirectional data transfer is being established. In recent years a system called the USB specification is also available which controls bidirectional data transfer between the host computer and the printer.
In conventional ink jet printing apparatus, to produce a color image with high color saturation without ink spreading, it has been a common practice to use dedicated paper that has an ink absorbing layer. Today, a new type of printing apparatus has emerged which, because of improvements made on ink, has a capability to print on plain paper that is widely used in large quantities as in printers and copying machines.
As a printing means for performing a color printing using two or more color inks, a so-called lateral array print head configuration is used in which nozzle groups (nozzle groups to be used), one for each color ink, are arranged one behind the other in the main scan direction so that each of the nozzle groups ejects ink droplets onto the same raster during the same scan.
In an ink jet printing apparatus using this lateral array print head configuration, there is a problem that since the next color ink lands on a print medium before the previously ejected color ink becomes fixed on the medium, these different color inks may spread and mix together. Particularly in plain paper, because of its ink soaking characteristic, different color inks easily spread one over the other and mix. If the intercolor bleeding occurs between a black ink and any other color ink, this clearly shows up, degrading the image quality significantly.
One method of addressing this problem uses a so-called longitudinal array print head configuration, in which the active nozzle groups assigned one to each different color are arranged such that at least one color ink nozzle group (for example, black ink nozzle group) is located at a position different from other color ink nozzle groups in a subscan direction (column direction). In this longitudinal array system, since the landing order (or overlapping order) of ink droplets ejected from different color ink groups does not change between a forward pass printing and a return pass printing, a resultant hue of the printed image does not change even if a bidirectional printing is performed. This allows a high-speed printing without having to execute complex image processing during the forward and backward passes. Further, since the time after the dots of one color land on specific rasters until dots of a different color land on the same rasters drastically increases, the quality of the printed image improves.
With the above system, it is possible to minimize undesired effects that the combined characteristics of plain paper and inks have on the image quality, thus realizing both high-speed printing and high-quality printing in one printing apparatus.
However, when the longitudinal array print head configuration is used, a bit map memory area in which image data is mapped (hereinafter referred to as a print buffer) needs to be made significantly larger than when the lateral array print head configuration is used. How large a print buffer area is required by the longitudinal array print head configuration will be explained in detail as follows by referring to the drawings.
FIG. 1 is a schematic diagram showing the longitudinal array print head configuration and its print buffer area. Here, it is assumed that the longitudinal array print head configuration has Y, M and C color ink heads each with (n+127) nozzles and a Bk ink head with (n+383) nozzles. These print heads of the four color inks are arranged in the order of Y, M, C and Bk in the main scan direction.
In this longitudinal array print head configuration, when the nozzle group used for ejecting Y ink performs printing in a raster range from an nth raster to an (n+127)th raster, the nozzle groups for M and C inks also print in the same raster range of nth to (n+127)th raster and the nozzle group for Bk ink prints in a raster range from an (n+256)th raster to an (n+383)rd raster.
As described above, print pixel data for each color is transferred from an external device (data source) such as a host computer to the printing apparatus one raster data or one line data at a time. Hence, in the longitudinal array print head configuration shown in FIG. 1, the print operation cannot start before the Y, M and C print data for nth to (n+127) raster and the Bk print data for (n+256)th to (n+383)rd raster are mapped into the print buffer.
If, for example, the Y, M and C print data for nth to (n+127)th raster are mapped into the Y, M and C print buffers and ready to be printed, the Bk print data for (n+256)th to (n+383)rd raster, which is to be fed to the active Bk nozzle group, is not yet mapped into the print buffer at this time, so that the active Bk nozzle group cannot start printing. It is thus necessary to keep the Y, M and C color nozzle groups waiting until the Bk print data for (n+256)th to (n+383)rd raster is mapped into the print buffer. Therefore, while the Bk print buffer needs only to have a memory capacity corresponding to 128 rasters of data from (n+256)th to (n+383)rd raster, the Y, M and C color buffers are each required to have a memory capacity capable of storing 384 rasters of data from nth to (n+383)rd raster. That is, the memory capacity for each color must be large enough to hold 384 rasters of print data, as shown in FIG. 1.
Here, if it is assumed that the resolution of the printing apparatus is 600 dpi, an image to be printed is A4 size and each raster has 4,800 pixels, then 384 rasters of print data require a 1,843,200-bit (=384 rasters×4,800 pixels) memory capacity to store. Similarly, Y print data, M print data and C print data each require a memory capacity of 1,843,200 (=384 rasters×4,800 pixels) bits, and the Bk print data requires a memory capacity of 614,400 bits (=128 rasters×4,800 pixels). Thus, summing the Y, M, C and Bk print data, a total of at least 6,144,000 bits of memory area is required.
On the other hand, the print buffer area referenced during one printing scan is 614,400 bits (=128 rasters×4,800 pixels) for each color, or a total of 2,457,600 bits for all Y, M, C and Bk colors, and it follows therefore that the minimum required memory capacity is less than half the 6,144,000 bits.
As can be seen from above, the conventional printing apparatus using the longitudinal array print head configuration requires a large-capacity print buffer (memory), which pushes up the cost of the apparatus. Further, since the time from the start of transferring print data from the host computer to the printing apparatus to the start of printing increases, the overall printing time also increases.
In the longitudinal array print head configuration in which a plurality of nozzle groups assigned one to each of different colors are arranged at different positions (offset) in the subscan direction, an offset transfer mode or mode may be employed. In this offset transfer mode the host computer performs offset processing to change the order of transfer of the print data according to the amounts of offset of the associated nozzle groups and successively transfers the print data offset in the subscan direction to the associated nozzle groups.
In this offset transfer mode, if an expensive high-performance host computer is used, there is no problem in executing the offset processing at high speed. With an inexpensive, low-performance host computer, however, performing the above-described data processing and the offset processing while transferring the print data is too large a burden and it may not be able to execute a smooth printing operation. Further, depending on the operating system (OS) used, the large burden of the offset transfer may render it impossible to offset and transfer the print data.
As described above, because in conventional printing apparatus the print data transfer mode is determined according to the nozzle group configuration, the printing operation may take a large amount of time or the data transfer may become impossible depending on the host computer used.